V Kappa 21 phenotypes, i.e. sequences, will be studied with emphasis on variation within the V Kappa 21 subgroups. The comparison between such variants and the germline genes coding for the V Kappa 21 subgroups will lead to an understanding of the mechanisms of somatic diversification. The genotypes of V Kappa 21 will be investigated in the NZB mouse by sequencing the germline V Kappa 21 and J Kappa genes. These gene sequences will be compared to NZB V Kappa 21 phenotypes and to the known sequences in the BALB/c mouse. The expression of V Kappa 21 will be studied by quantitation of these light chains in serum and the evolution of these light chains will be followed by studying serum expression and enumeration of V Kappa 21 genes in inbred and wild mice. The expression of light chains at the molecular level will be studied by analysis of the structure of V Kappa 21 genes and other rearranged, but not transcribed V Kappa genes will be studied to clarify the events resulting in V gene rearrangement and the molecular requirements for V gene expression.